Encapsulation of nitronium perchlorate employing ammonia to form ammonium perchlorate

ABSTRACT

1. A PROCESS FOR ENCAPSULATING NITRONIUM PERCHLORAATE COMPRISING THE STEP OF FLOWING AMMONIA DILUTED WITH AN INERT GAS OVER NITRONIUM PERRCHLORATE PARRTICLES.

3,728,169 ENCAPSULATION F NITRONIUM PERCHLO- RATE EMPLOYING AMMONIA TOFORM AMMONIUM PERCHLORATE James P. Diebold, China Lake, Calif.-,assignor to th United States of America as represented by the Secretaryof the Navy N0 Drawing. Filed Mar. 18, 1963, Ser. No. 266,105

Int. Cl. C06d /06 U.S. Cl. 149-5 5 Claims Needle valve (Rotameter)Flomneter is necessary. Literature surveys show that the work in thepast toward passivation of nitronium perchlorate has been directedtoward encapsulation with an inert plastic binder or with an aluminumcoating. These passivation techniques have not proved entirelysatisfactory. One disadvantage is that there is a decrease intheoretical specific impulse (I due to the use of essentially inertorganic encapsulating compounds. Other disadvantages'include anon-continuous coating which does not effect complete 3,728,169 PatentedApr. 17, 1973 volving the passing of dry ammonia gas over particles ofnitronium perchlorate.

An object of the present invention is to provide an encapsulatingtechnique for rendering high energy solid fuels and oxidizers inert toeach other and to the binder material until such time as the system isignited.

Another object is to provide a method for encapsulating nitroniumperchlorate for use in the propellant field.

. A further object is to provide nitronium perchlorate particles havingcontinuous coatings which isolate the nitronium perchlorate from thepropellant binder.

Other objects and advantages of this invention will become readilyappreciated as the same become better understood by reference to thedetailed description.

The present invention is essentially concerned with encapsulation ofnitronium perchlorate (NP) with ammonium perchlorate (AP) by a reactionprocess comprising the passing of dry ammonia gas (NH over particles ofnitronium perchlorate whereby the ammonia reacts with thenitroniumperchlorate to form ammonium perchlorate on the surface of eachparticle.

A simplified diagram of the process is as follows:

Annnonia.

gas y) Helium Needle valve Dryer filled. with molecular sieves Flowmeter(Rotameter) Heat-exchanger cooled. by ice Vent;

U-tube reactor containing nitronium perchlorate particles (cooled by icebath) Exhaust The equipment used consisted primarily of glasswareconnected by rubber hoses. To control the flow rates of the helium (usedas an inert diluent) and ammonia, micrometervalves were installed andcalibrated. Later it was found that with these valves one could notcontrol the flow rates as accurately as was desired, and rotameters wereinstalled to augment the flow control. Due to the exothermic nature ofthe reaction, the incoming gases and the reaction vessel (a glassU-tube) were cooled by ice baths. The helium was found to be wet and itwas necessary to install a drying unit filled with molecular sieves.

A series of experiments were conducted using nitronium perchloratepellets weighing about 3 milligrams each and ammonia gas ranging inconcentration from about 5% to 60%. The initial reaction temperatureswere maintained at about 34 F., but was also found that the reactionwould occur at initially ambient conditions. The following examples areillustrative of the process above.

3 EXAMPLE I 4 pellets of nitronium perchlorate weighing about 12milligrams were placed in the reactor (a glass .U-tube) which was cooledby an ice bath. A 60% concentration of ammonia gas was passed over thepellets. The flowrate for the ammonia was about 9.6 cc. per second, andthe dried helium about 6.2 cc. per second through the inch diameterglass tubing reactor. The reaction was continued for about 8 minutes at34 F. The pellets were then tested for encapsulation by casting them ina polyurethane binder. This binder is not compatible with nitroniumperchlorate and is, in fact, incompatible with some forms ofplastic-encapsulated nitronium perchlorate. One pellet of thisexperiment was coated and found to be compatible with the binder. Two ofthe pellets were completely reacted with the ammonia and one pellet waslost during the reaction when it dissolved.

EXAMPLE II Following the same procedure outlined above three nitroniumperchlorate pellets were treated with an 8% concentration of ammoniadiluted with dry helium. The flowrate of the gases was controlled toabout 6 cc. per second for 15 minutes at an initial reaction temperatureof 34 F. One pellet was completely encapsulated and did not react withthe polyurethane binder.

It is postulated that the process may be represented by the followingmultistep equation:

It was found that pellets of the same run would not necessarily reactsimilarly. Liquid formation was observed during many of these runs. Ifthe pellets were to have been wetted during the first stage of theprocess before the ammonia had diffused throughout the pellet, then thisliquid would act as a barrier to further ammonia penetration. Thisapparently would limit the reaction to the surface of the pellet to formthe desired coating.

The liquid formed by the overall reaction was the subject of muchtheorizing. In some experiments this liquid did not necessarily collectentirely around all the pellets. There was observed occasionalcondensation of the liquid down-stream from the pellets. This liquidwould eventually solidify into a white material. The white material wasobserved to be inert with respect to the binder. The most plausibleexplanation for this phenomena is that the liquid formed was anhydrousperchloric acid (HClO and nitric acid (HNO The solidification processwould then be the reaction of the basic ammonia (NH with the acids,shown in Equation 3 above.

Since it appears that a multistep process is probable, if conditions arevaried such that one reaction is favored over another, the resultantproducts will also be varied. It was observed that Equations 1 and 2above, were favored at the higher ammonia (NH concentrations asevidenced by a considerable liquid formation. At lower concentrations ofammonia (see Example II) Equation 3 was favored because there was noobservable liquid formation. This would indicate that the liquid wasreacting with the ammonia at a rate comparable to the rate of formationof the liquid.

It should be noted that nitronium perchlorate and ammonium perchloratehave dilferent crystal structures and therefore a disordered crystal mayresult. A disordered crystal would have different decompositioncharacteristics than those of the pure substances. It is thought thatammonium perchlorate was possibly produced in the cubic form by thisreaction, as Differential Thermal Analysis did not reveal the ammoniumperchlorate phase change from rhombic to cubic.

The variations in results of some of the experiments carried out intrying to perfect the present process could be due to varying porosity,possible variation of composition'on the pellets, and localconcentration of the am- 5 monia in the gas stream. If the pellets wereporous, am-

monia would have an opportunity to diffuse throughout the pellet causingcomplete reaction with the nitronium perchlorate. I

' A slight variation in the procedure described in Examples I and II isproposed below:

EXAMPLE III After placing the nitronium pellets in the reactor the firststep would be to pass a high concentration of ammonia to 60%) over thepellets to form a liquid (anhydrous perchloric acid). This would befollowed by a helium purge to rid the system of ammonia and reactionproducts and to reduce the temperature of the pellets. Then a lowconcentration of ammonia (less than 5%) would be passed over the pelletsto react with the liquid on the surface forming the desired ammoniumperchlorate coating. This reaction may be represented by the followingequation:

(1) NH (high concentration) +NO ClO., H O+oxides of nitrogen (2) NH (lowconcentration) +HCl NH ClO In another series of experiments acrystalline form of nitronium perchlorate was used which had a particlesize of 3 to 5 microns. The reaction time was about minutes at about 34F. Using a concentration of 40% ammonia, some of the nitronium crystalsreacted to form a liquid which cemented the remaining crystals togetherto form a solid mass. In the concentration range between 5% and 40%reaction took place which did not form a solid mass. However, there wasobserved a definite trend of less agglomeration with lower ammoniaconcentration. Using 4% ammonia, there was no obvious physical orchemical change in the material. At ice bath temperatures, 4.7% ammoniaappears to be about the lowest concentration that will promote areaction. The observation of the effect of varying ammoniaconcentrations are shown in Table I below:

Appearance of product in moist Percent NH in He stream atmosphere 1Fuming indicates presence of N 020104.

In order to determine the conditions favorable for the desired reactionin which the coating was formed, the solid products were analyzed. Thequalitative tests showed that ammonium nitrate and ammonium perchlorateare possible reaction products as shown in Equation 3 herein.

Ammonium nitrate may not necessarily be present as the casting thepellets coated in Example I above into the binder, that when some of thepellets were prodded, the coating was fractured exposing a mushyinterior indicating the reactions of NH with most of the NP in thepellet.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A process for encapsulating nitronium perchlorate comprising the stepof flowing ammonia diluted with an inert gas over nitronium perchlorateparticles.

2. A process for coating nitronium perchlorate particles with ammoniumperchlorate which comprises (a) flowing about a 60% concentrationammonia gas over nitronium perchlorate particles until said particlesare wetted; and

(b) continuing the flow of ammonia over said particles for about 8minutes at about 34 F. until the wetting liquid has disappeared aroundeach particle.

3. A process for coating nitronium perchlorate with ammonium perchloratewhich comprises the following multistep reaction:

References Cited UNITED STATES PATENTS 3,006,743 10/1961 Fox et al.14919 3,535,172 9/1969 Bieber et a1. 1497 BENJAMIN R. PADGET'I, PrimaryExaminer US. Cl. X.R.

ll7100 B; 149-19, 423-386, 476

1. A PROCESS FOR ENCAPSULATING NITRONIUM PERCHLORAATE COMPRISING THESTEP OF FLOWING AMMONIA DILUTED WITH AN INERT GAS OVER NITRONIUMPERRCHLORATE PARRTICLES.